The present invention relates to a polycrystalline silicon sputtering target produced by a melting method.
Accompanied by progress in miniaturization and improvement of the precision of LSI, sputtering targets as raw materials for forming thin films have been increasingly demanded to have higher purity and higher stability (reduction in frequency of occurrence of arcing and particles) during sputtering.
Also in sputtering target of silicon (Si), similarly, it is demanded to reduce not only the amount of metal impurities, which adversely affect LSI, but also the amounts of compounds composed of silicon and gas components, such as silicon oxide, silicon nitride, and silicon carbide, which cause occurrence of arcing and particles during sputtering.
Hitherto, the Si materials that are used for Si sputtering targets and the production methods can be classified into three types. One of them is a monocrystalline ingot that is produced by a Czochralski method (CZ method) or a floating zone method (FZ method), and the ingot is used by cutting to about a target thickness.
The second is a polycrystalline silicon block produced through a sintering process by producing a fine silicon powder and sintering the powder by, for example, hot-pressing under high temperature and high pressure conditions (see Patent Documents 1 and 2).
The third is a polycrystalline silicon ingot produced through a melting method, which is a usual method of producing polycrystalline silicon for solar cells, by melting silicon once in a crucible and allowing growth in unidirectional solidification.
Selection from these three types of Si materials is performed depending on the size and price of a sputtering target and the purpose of a thin film to be formed.
In the case of monocrystalline silicon, about 300 mm is the maximum diameter of generally available ingots for producing silicon wafers having a diameter of 300 mm, which is the current majority, and there is a restriction in production of a sputtering target having a diameter larger than this.
Although prototype ingots having a diameter of 450 mm have been recently produced towards a shift to 450 mm wafers, the state is that there are still problems in price and supply thereof.
The sputtering target corresponding to a 300 mm silicon wafer usually needs a diameter of 420 mm or more, and it is believed that a 450 mm wafer needs a target having a diameter of about 600 mm. Thus, monocrystalline silicon, even if it has a high purity (11N) or an excellent sputtering property, has a basic problem of incapable of corresponding to large diameter products.
On the other hand, in polycrystalline silicon produced through a sintering process, raw materials are required to be formed into fine powders once, and the powder surfaces are progressively oxidized in the process. Even if deoxidization is performed (see Patent Documents 1 and 2), the amount of oxygen is large compared to the amount thereof in cases of monocrystals or the silicon material for the melting method, and the polycrystalline silicon produced through a sintering process has a problem of readily causing occurrence of arcing during sputtering.
In addition, since the polycrystalline silicon produced through a sintering process is contaminated with impurities in the pulverization step, it is difficult to obtain high purity compared to silicon in other methods, and the purity, excluding gas components (C, N, and O), is about 5N to 6N.
However, the sintered silicon has a higher bending strength than that of other Si materials and is therefore hardly broken even if bending stress occurs during sputtering. Since the size of crystal grains is significantly smaller than that of silicon produced through a melting method, a homogeneous thin film can be advantageously formed.
A polycrystalline silicon ingot produced through a melting method has a demerit that the crystal grains coarsen, resulting in a decrease in bending strength. However, the polycrystalline silicon ingot is produced for use in solar cells and is actually widely used in large-sized products of 840 mm square or more. The polycrystalline silicon ingot can satisfy a purity (excluding gas components) of 6N to 7N, which is high for polycrystalline silicon, and has a merit of being relatively inexpensively available.
From these backgrounds, in Si targets having a diameter of 420 mm or more corresponding to a 300 mm wafer, which is the current majority, polycrystalline silicon produced through a melting method is widely used.
However, it has been revealed that a sputtering target produced from a polycrystalline silicon ingot prepared by melting silicon in a conventional silica crucible and unidirectionally solidifying the silicon from the bottom of the crucible cannot obtain sufficient characteristics for recent new uses of Si thin films.
The reason thereof is as follows. Silicon nitride (Si3N4) is applied to the inner wall of a silica crucible before subjecting silicon to melting in order to prevent silicon from reacting during melting and burning during solidification. However, the silicon nitride becomes mixed into the molten silicon or precipitates during a step of cooling the silicon after being once subject to melting. As a result, as shown in FIG. 1, acicular or annular silicon nitride is generated in the silicon structure, and it is likely that the presence of such silicon nitride causes occurrence of arcing or particles during sputtering.
For the use in solar cells, the presence of foreign substances composed of such silicon nitride (Si3N4) does not affect the conversion efficiency of sunlight and, therefore, does not conventionally cause a problem. However, it was revealed that in sputtering targets, the presence of the contaminant described above causes a major problem.
If the carbon concentration in a silicon material before being melted is high, silicon carbide is produced when silicon is subject to melting. As a result, petaloid silicon carbide is generated in the silicon structure as shown in FIG. 2, and it has been revealed that the presence of silicon carbide similarly causes occurrence of particles. In FIG. 2, acicular or annular silicon nitride (Si3N4) is also observed.
Thus, even if a polycrystalline silicon sputtering target is produced by a melting method, which has an advantage in manufacturing cost, the target has some problems, and the problems are desired to be solved.
Patent Document 1: JP H05-229812 A
Patent Document 2: JP 2004-289065 A